The present invention relates to a novel class of soluble organic nitrogen compounds which produce a highly visible color when dissolved in organic liquid solvents. An original color ranging from blue to red is produced in the organic solution depending upon the relative polarity of the particular organic liquid solvent and which changes to a yellow color if the particular liquid organic medium becomes acidified. The original color can be reproduced in said organic liquid medium upon conversion to a neutral or basic pH by such conventional means as adding a soluble base or bubbling gaseous ammonia through the medium.
Various acid-base indicator dyes are known to produce a color change in both aqueous and non-aqueous liquid medium upon acidification. Cresol Red is a well known organic dye which changes color when acidification occurs in a particular solution. A more recently discovered organic nitrogen dye exhibiting different colors in aqueous and pyridine solutions has been reported by Brooker, Keyes & Heseltine in Journal of the American Chemical Society, Volume 73, page 5350 (1951). The particular dye is reported to have the chemical formula ##STR1## and the color shift is attributed to different resonance forms of the dye molecule.